The present invention relates to a method of producing a ball joint and its housing, wherein an internal thread is formed in the inner surface of the arm portion of the ball joint.
An example of conventional ball joints is shown in FIG. 7.
The conventional ball joint 51 shown in FIG. 7 has a housing 56, in which a generally spherical ball head portion 53 is enveloped, with a bearing seat (not shown) disposed between the spherical ball head portion 53 and the housing 56. The ball head portion 53 is provided at an end of a generally rod-shaped ball stud 52. The bearing seat is formed of synthetic resin. The housing 56 has a generally cylindrical body portion 55. The body portion 55 is provided at one end thereof with an opening from which a stud portion 54 of the ball stud 52 is projected. An arm portion 58 extends from the cylindrical outer surface of the body portion 55 of the housing 56 as an integral body with the body portion 55. By screwing an external thread (not shown) in the arm portion 58, another ball joint or the like can be coupled to the housing 56. The arm portion 58 has a generally cylindrical shape in which an internal thread 57 that permits an external thread to be screwed therein is formed. The production process of the arm portion 58 of the housing 56 calls for forming an intermediate molding including a cylindrical arm-to-be portion which has nearly the same dimension as that of the arm portion 58 and integrally projects from the cylindrical outer surface of the body portion 55 and forming the aforementioned internal thread 57 in the cylindrical inner surface of the arm-to-be portion of the intermediate molding by inserting a tap 61 into the arm-to-be portion while rotating said tap 61. Said tap 61 is provided with an external thread 60 formed along the cylindrical outer surface of the tap 61.
However, the structure shown in FIG. 7, wherein an internal thread 57 is formed by threading the arm-to-be portion of the intermediate molding with a tap 61 that has an external thread 60, enables the efficient processing, because it requires a considerably long machining time to perform reliable and stable formation of an internal thread 57 having a given pitch. Furthermore, great frictional force is generated between the tap 61 and the arm-to-be portion during threading, which frictional force wears the tap 61 and reduces its life span.
In order to solve the above problems, an object of the present invention is to provide a method of producing a ball joint and its housing, of which arm portion that has stable characteristics and is provided with an internal thread formed in its cylindrical inner surface can easily and quickly be formed.
The present invention provides a method of producing a ball joint housing which has an arm portion projected therefrom and is adapted to support a ball stud, said method calling for inserting a tap that has an external thread formed along its cylindrical outer surface into a hole formed in an intermediate molding, said hole having a cylindrical inner surface with a diameter greater than the diameter of said tap, and forming an internal thread in the cylindrical inner surface of said hole by narrowing said intermediate molding, in which the tap is inserted, by cold forging. Compared with a method that calls for forming an internal thread by threading with a tap having an external thread, the method of the invention enables the easier and quicker formation with an increased efficiency. As the invention also prevents wear or other damage to the tap, it is free from the problem of a damaged tap impairing formation of an internal thread having given dimensions, and therefore facilitates formation of the arm portion that has stable characteristics.
A method of producing a housing of a ball joint according to another feature of the present invention is characterized in that a pair of pressure dies respectively provided with concave faces, which are formed at locations opposite each other and are in the shape of a half cylinder corresponding to the half circle of the outer surface of the arm portion with a curvature corresponding to that of the cylindrical outer surface of the arm portion, function to narrow said intermediate molding by sandwiching the intermediate molding between said concave surfaces and, in this state, applying pressure to the intermediate molding. This feature enables the easy formation of an arm portion having stable characteristics without deviation of the center axis.
A method of producing a housing of a ball joint according to yet another feature of the present invention calls for forming, prior to narrowing the intermediate molding, a pair of flat faces cylindrical outer surface of the portion of the intermediate molding in which said hole is formed, said flat faces formed in such a manner as to diametrically oppose each other and extend either in parallel with or at an angle from, except perpendicularly, the directions in which the pressure is applied by said pressure dies. As a result, the method is effective in preventing formation of a burr or other similar defects, which would otherwise be formed on the cylindrical surface of the outer rim of the hole of the intermediate molding by flow of the surplus material in the course of narrowing process performed by the pressure dies. This feature thus eliminates the need of cutting work or other work to remove a burr and, consequently, prevent a change of the volume of the housing that would otherwise be caused by removal of a burr. Therefore, the invention ensures the easy and reliable formation of a housing having stable characteristics.
A method of producing a housing of a ball joint according to yet another feature of the present invention is characterized in that the intermediate molding is formed of an aluminum alloy. Because of this feature, the invention facilitates the narrowing of the intermediate molding in the state where the tap is inserted in the hole. Therefore, the invention is in improving the manufacturability of the housing by ensuring the easy formation of a housing, even if the housing will be used as a component that will have to bear a heavy load, such as a component of a vehicle.
A ball joint according to the present invention includes a housing formed by the method of producing a housing of a ball joint described above so as to have an opening and an inner chamber that directly communicates with the opening, a ball stud having a ball head portion and a stud portion positioned such that the ball head potion is contained in the inner chamber of the housing and that the stud portion is formed as a contiguous, integral body with the ball head portion and projected from said opening of the housing, and a bearing seat having an insertion hole and contained in the inner chamber of the housing so that the ball head portion of the ball stud is slidably enveloped in the bearing seat and that said stud portion projects from the insertion hole. As a result of using a housing that has efficiently and easily been formed with stable characteristics by the method of producing a housing of a ball joint described above, the manufacturability of the ball joint is improved.